The present invention relates to a method of manufacturing a container cap having a mark or indicia concealed in it, the container being, for example, a soft drink bottle with a prize, having a cap with a winning mark concealed in it.
One typical method for manufacturing such a mark is the so-called "in-shell molding" method. In this method, a mark is applied on the inside face of the top panel of a cap shell by printing or like processes, a molten material of a synthetic resin is placed on the mark-printed face of the top panel and the molten material is subsequently embossed to form into a gasket member concealing the mark.
This in-shell-moulding method requires high speed supply of a molten resin to each cap shell in a predetermined amount. This necessitates a special extrusion device which is expensive, and hence this method is disadvantageous in that it is high in equipment cost.
Furthermore, this conventional method necessitates an additional die which is adapted to be inserted into a cap shell so as to surround the outer periphery of a gasket to be formed during embossing for preventing the molten resin from being squeezed out through the clearance between the cap shell and the embossing die inserted in it. The gasket completed according to this method has a clearance between its outer periphery and the skirt of the cap shell due to the insertion of the additional die. Thus, this method has another disadvantage in that the cap can provide insufficient sealing to a mouth of a container over which it is fitted, and hence a special design is required for the mouth of the container in order to provide reliable sealing to it.